Meter, Meter on the Wall
Question from the June 13 issue
(Exam level: CBRE)
The analog DC ammeter with an actual full-scale value of 10 milliamperes in your transmitter that measures 1 ampere of plate current has failed and the only linear meter that fits in the same space available is a 1 mA unit. Could you use this available meter as a replacement?
a. Yes, with a series resistor of 10 k
b. Yes, with a series and parallel resistor both 10 k
d. No because the scale is set by the coil wind count
e. Yes, with a select parallel resistor across the meter contacts
SBE certification is the emblem of professionalism in broadcast
engineering. To help you get in the exam frame of mind, this column poses
typical questions. Although similar in style and content to the exam questions,
these are not from past exams nor will they be on future exams in this exact
assume that the meter has no resistance, which will simplify our discussion for
the moment. We’ll come back to this, I promise.
Fig. 1: Basic Meter Circuit With
Series Resistor for Voltage Measurements
In the ultimate analysis, all analog DC meters are actually displaying
current flowing through the meter coil that deflects the pointer. With that in
mind, answer (a) would really be creating a meter that responds to voltage.
In Fig. 1, we have
voltage impressed between the top of the series resistor and the negative
terminal of our meter. The values shown of 10 volts and a series resistor of 10
k would cause a current flow of 1 mA or full scale on our replacement meter.
When you buy a voltage meter, this series resistor is inside already.
To keep the parts acquisition count down, most transmitter manufacturers
use the same model milliammeter in every position and a variety of this series
resistor scheme to create the appropriate scale values for voltages. For
example, a full scale of 3000 volts using a 1 mA meter would require a 3 megohm
series resistor. Preferably, a precision resistor would be used in this
instance to enhance accuracy.
The answer (b), with at least one of the 10 k resistors being in series,
would make it impossible to use this meter as an ammeter to represent 1 ampere
as that 10 k resistor would drop any impressed voltage notably. If you throw in
the voltage divider action of that shunt 10 k resistor, the simple reality is
that this arrangement is just a more complicated “voltage variable” version of
The answer (c) is reserved for someone who either has no knowledge of
the topic or didn’t get a good night’s sleep the night before the test! One
should always come to SBE exams well rested and with a clear mind. This is the
last choice to go with until all other options have been eliminated.
The wind count mentioned in answer (d) is a factor in creating the small
DC resistance of the meter (that we decided above to discount momentarily), but
is just one element involved in determining the full scale value.
HOW IT WORKS
To answer the question, (e) is correct, as we can increase the value
displayed on this 1 mA meter to be calibrated to 1 amp FS with a parallel
resistor, known as a shunt, across the meter contacts.
Fig. 2: Shunt Resistor for
How do we determine this shunt resistor, which will route all current
above 1 ma around the meter leaving just an analogous, representative current
to be displayed on the meter?
Meters do have resistance, and since we now need to consider parallel
paths for current flow, we really need to know the actual value of the meter
coil resistance. A typical value for a 1 mA meter is about 200 ohms. If you
don’t know the meter resistance, you’ll have to ascertain what it is.
In most cases, it is easier to calculate (or measure) the voltage across
the meter movement for full-scale display. If you don’t know the resistance, it
can be measured with a digital multimeter. The current from most digital
multimeters is low enough not to cause damage to the meter, but the pointer may
swing rather violently. Connect with reverse polarity to minimize the risk of
bending the pointer.
Fig. 2 shows our 1 mA meter with a shunt resistor across the meter
contacts. We’ve assumed our meter’s internal resistance is 200 ohms. The
voltage across the meter and the shunt is in parallel so Vs (=
voltage across the shunt) is equal to Vm (= voltage across the meter
winding). But we also know that Vs must be equal to the current
through the shunt times the shunt resistance and that Vm must be
equal to the current through the meter times the meter resistance.
In shorthand here are the relationships:
Vs = Vm; and
Is × Rs = Im × Rm
We can solve for Rs (shunt resistance) by
rebalancing the equation:
Fig. 3: Variable Shunt Resistor
Rs = (Im × Rm) / Is
Substituting Iin – Im for Is
Rs = (Im × Rm)/(Iin
Let’s give it a try using that typical 200 ohm value for the
ammeter resistance value.
(0.001 amp × 200 ohms) /
(1 amp – 0.001 amp)
Rs = 0.2 volt / 0.999 amps
Rs = 0.2002002 ohms
At this low a resistance value, the pragmatics of creating a pure and
exact resistance at that small value are important, as even stray resistance of
the connection wires can affect measuring accuracy. Special wire and bar stock
are made just for the purpose of achieving small resistance values with high
current handling. Most often a manganin and/or constantan alloy that exhibits a
low coefficient of resistance change with temperature is used for highest
accuracy across the current range of interest. Most full-line meter
manufacturers usually have shunts available off the shelf to multiply their
ammeters scales in standard values.
In the example above, if ultra accuracy is needed, a 2 ohm shunt would
be used and a small value “trim” pot can be added in series with the meter (see
Fig. 3). Small in our case would be a 25 to 50 ohm (usually multi-turn) pot.
Referenced against laboratory grade instruments, the trim pot would be adjusted
to match and double checked at full scale as well as several intermediate
values of current.
The next SBE certification exams will be given
in the local SBE chapters Nov. 2–16. Closing date for signing up is Sept. 14.
If you are interested and ready to take the exams, we strongly suggest that you
sign up ASAP, as the next exams are scheduled for Feb. 8–18, 2013.
Charles “Buc” Fitch, P.E., CPBE, AMD, is a frequent
contributor to Radio World. Graphics were drawn by Victor Osorio, the full-time
CE at HRRZ AM and HRKD FM in Juticalpa, Olancho, Honduras. Reach him at
Missed some Certification Corners
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articles under the Columns tab at radioworld.com.
The Mismatch Patch
(Exam level: CBNT)
A cabling impedance mismatch can be caused by which of the following?
a. Using RJ45 connectors on Cat-6 cable.
b. Running cable in an overhead cable tray near electrical conductors.
c. Nicking cable conductors when stripping.
d. Mixing shielded and unshielded twisted pair cable in the same segment.
e. Not following ITE/EIA/IEEE standard UTP-101 regards wire twist format (i.e. left overhand)